What are the normal ABG ranges?

pH 7.35–7.45, PaCO₂ 35–45 mmHg, HCO₃⁻ 22–29 mEq/L, PaO₂ 80–100 mmHg. Hypoxemia: mild 60–79, moderate 40–59, severe <40.

How do I use Winter’s formula?

Expected PaCO₂ ≈ (1.5 × HCO₃⁻) + 8 ± 2. If actual PaCO₂ is higher, suspect additional respiratory acidosis; if lower, respiratory alkalosis.

How do I distinguish acute vs. chronic respiratory acidosis?

Acute: HCO₃⁻ ↑ ~1 mEq/L per 10 mmHg ↑ PaCO₂. Chronic: HCO₃⁻ ↑ ~4 mEq/L per 10 mmHg ↑ PaCO₂. (Opposite direction for respiratory alkalosis.)

PulmTools

ABG Interpretation

Mastering ABG Analysis: A Step-by-Step Guide for Students & Clinicians

Q: How do I use Winter’s formula?

Expected PaCO₂ ≈ (1.5 × HCO₃⁻) + 8 ± 2. If actual PaCO₂ is higher, suspect additional respiratory acidosis; if lower, respiratory alkalosis.

Q: How do I distinguish acute vs. chronic respiratory acidosis?

Acute: HCO₃⁻ ↑ ~1 mEq/L per 10 mmHg ↑ PaCO₂. Chronic: HCO₃⁻ ↑ ~4 mEq/L per 10 mmHg ↑ PaCO₂. (Opposite direction for respiratory alkalosis.)

2025-08-29 · ~8 min read

Arterial blood gas (ABG) analysis evaluates acid–base balance, ventilation, and oxygenation. This guide shows a reliable, repeatable workflow you can use at the bedside and while studying.

Normal Ranges

pH 7.35–7.45
PaCO₂ 35–45 mmHg
HCO₃⁻ 22–29 mEq/L
PaO₂ 80–100 mmHg

Hypoxemia: Mild 60–79, Moderate 40–59, Severe <40. Hyperoxemia: >100 (consider FiO₂).

Step 1: pH — Acidic or Alkalotic?

pH < 7.35 → acidosis. pH > 7.45 → alkalosis. Normal pH can still mean a compensated disorder.

Step 2: Determine the Primary Disorder

Respiratory Acidosis

↑ PaCO₂ (hypoventilation): COPD/asthma exacerbation, pneumonia, CNS depression (opioids/sedatives), neuromuscular weakness.

Respiratory Alkalosis

↓ PaCO₂ (hyperventilation): anxiety/pain, fever, early sepsis, pregnancy, CNS injury.

Metabolic Acidosis

↓ HCO₃⁻: DKA, lactic acidosis (shock/hypoxia), renal failure, diarrhea, salicylates (late).

Metabolic Alkalosis

↑ HCO₃⁻: vomiting, NG suction, diuretics, hypokalemia, excess antacids.

Step 3: Check Compensation

Compensation aims to normalize pH but rarely fully corrects it. Use expected relationships to spot additional processes.

Winter’s formula (metabolic acidosis): Expected PaCO₂ ≈ (1.5 × HCO₃⁻) + 8 ± 2.
Metabolic alkalosis: PaCO₂ ↑ ~0.7 mmHg per +1 mEq/L HCO₃⁻ (±5).
Respiratory acidosis: acute HCO₃⁻ +1 / +10 PaCO₂; chronic +4 / +10.
Respiratory alkalosis: acute HCO₃⁻ −2 / −10 PaCO₂; chronic −5 / −10.

Step 4: Evaluate Oxygenation

Classify hypoxemia by PaO₂, but always consider FiO₂ and shunt. For lung injury/ARDS, track PaO₂/FiO₂; an elevated A–a gradient suggests V/Q mismatch, diffusion issues, or shunt.

Oxygenation Index A–a Gradient

Mixed Acid–Base Disorders

If actual values deviate from expected compensation, suspect a mixed process. Example: pH 7.25 / PaCO₂ 25 / HCO₃⁻ 10 → metabolic acidosis with concurrent respiratory alkalosis (e.g., sepsis, salicylates).

Practice Case

ABG: 7.18 / 75 / 26 / 51

pH low → acidemia
PaCO₂ high → respiratory acidosis
HCO₃⁻ not sufficiently elevated → uncompensated
PaO₂ 51 → moderate hypoxemia

Answer: Uncompensated Respiratory Acidosis with Moderate Hypoxemia

Practice with the ABGenius →

FAQ

What if the pH is “normal” but PaCO₂/HCO₃⁻ are abnormal?

That suggests a compensated disorder. Use compensation rules to see if it’s appropriate or mixed.

Should I prioritize pH or the primary driver first?

Start with pH (acidemia/alkalemia), then identify the primary driver by looking at PaCO₂ vs HCO₃⁻.